The present invention relates generally to the field of semiconductor processing and, more particularly, to arsenic gas phase doping for integrated circuit fabrication.
Arsenic and phosphorus are both used as N-type dopants in semiconductor structures, such as silicon wafers. For certain applications, arsenic is particularly desirable due to the larger size of the arsenic atom and its consequently lower diffusivity as compared to phosphorus. One disadvantage of arsenic is that it is very toxic in both its metallic state and its oxide.
Although the chemical can be confined within a closed processing system during the doping process, at the end of the process the processing system needs to be opened to unload the wafers. At this point, outgasing of arsenic or arsenic compounds from the wafer and the reactor can occur, potentially causing toxic leakage into the clean room environment. As a consequence, stringent safety measures are required.
One manner of reducing the risk of toxic leakage is to unload substrates at such a low temperature that outgasing is insignificant when opening the system. This may mean reducing the temperature from the doping temperature (around 900xc2x0 C.) to within the range of 300xc2x0 C. to 500xc2x0 C. Unfortunately, it takes a lot of time and energy to cycle a hot wall furnace every run through such large temperature differences. Hot wall batch furnaces are generally preferred for superior throughput for gas phase doping systems, but cycling through such temperature swings would greatly reduce the throughput of such systems.
U.S. Pat. No. 5,324,684 to Kermani et al. describes the use of a cold wall radiantly heated reactor for gas phase doping processes. Cold wall reactors can be cycled through temperature differences much more quickly. The disadvantage of a cold wall system, however, is that reactive gases adsorbed on the cold wall of the system are not easily purged away. A xe2x80x9cmemory effectxe2x80x9d from residual gases or residues thus creates a safety risk when the system is opened for wafer transfer. Furthermore, if the wafers are inserted in an adjacent processing system for a subsequent treatment at elevated temperatures, there is again the risk of outgasing.
It is accordingly an object of the present invention to omit the above and other disadvantages and to provide an arsenic gas phase doping process that allows safe operation without elaborate safety measures.
In accordance with one aspect of the invention, a method is provided for gas phase doping a semiconductor with arsenic. The method includes exposing the semiconductor to a non-oxidizing, arsenic source gas within a reaction chamber. Thereafter, prior to opening the reaction chamber, a sealant layer is formed over the semiconductor structure. The sealant layer inhibits outdiffusion of arsenic when the substrate carrying the semiconductor structure is unloaded from the chamber, even at relatively high temperatures.
In the illustrated embodiments, the sealant layer can be formed by oxidation, nitridation or chemical vapor deposition. Forming the sealant layer can be conducted prior to, during or after cooling the substrate to an unloading temperature. Preferably, a gettering step is conducted after gas phase doping and prior to forming the sealant layer, such as by exposing the substrate to HCl vapor.